The origin of the depletion of moderately volatile elements (MVEs) in the Moon.
The prevailing view for the formation of the Moon is that it was derived from a disk created by the collision of a Mars-sized body with the protoEarth (the so-called “giant impact” hypothesis). Although the hypothesis can explain many features of the Moon, it cannot readily explain why the Moon is depleted in MVEs (e.g, K, Rb, Cu, Zn and Ga) relative to Earth. We investigated this question using a new isotope tracer of Rb, as this element is very sensitive to volatilization processes, but is largely immune to lunar core formation. We measured high-precision Rb isotope data on selected lunar samples recovered by the Apollo missions. The new data showed that lunar volatile elements were very likely lost to the Earth through viscous drainage from the protolunar disk at a vapor saturation level of ~99%; this is a natural outcome if the viscosity of the vapor disk was controlled by magnetorotational instability. The model quantitatively explains the depletion and isotopic fractionation of MVEs in the Moon. Read more
Iron UV photo-oxidation on the early Earth and Mars.
Banded iron formations (BIFs) contain appreciable amount of ferric iron (Fe(III)), but the mechanism by which ferrous iron (Fe(II)) was oxidized to Fe(III) in an atmosphere that was globally anoxic is highly debated. We investigated the possibility of UV photo-oxidation as this oxidation mechanism. Indeed, the oxygen-free atmosphere in the Archean could have allowed about five times more UV light to reach Earth’s surface, oxidizing Fe(II) in the top layers of the oceans. We investigated this hypothesis by designing and performing laboratory photochemical experiments involving the oxidation of Fe(II) solutions by UVs under anoxic condition. By measuring the Fe and oxygen isotopic compositions of the run products at ultra-high precision, and comparing the results with isotopic signatures of natural BIFs, we showed that UV photo-oxidation was a viable pathway to BIF formation. Based on our experiments we also estimated the quantum yield of Fe UV photo-oxidation, which was used to test and model UV photo-oxidation on Mars to produce Martian “blueberries” (hematite spherules). We found that photo-oxidation in shallow water ponds under ancient Martian conditions could be fast enough to produce the large quantities of Martian “blueberries”, demonstrating the importance of UV photo-oxidation on shaping the surface of early Mars and Earth. Read more
Formation of hematite spherules in Hawaii, an analogue for Mars.
Understanding how Martian “blueberries” were formed is essential to reconstructing the conditions that prevailed on early Mars. Given the unavailability of samples, studying terrestrial analogues provides an opportunity to obtain a more comprehensive picture of their formation. Hematite spherules that share many similarities with Martian “blueberries” have been found on Mauna Kea volcano, Hawaii. We performed a systematic Fe isotope study of Hawaiian spherules and associated lithologies. The work examined samples that experienced various types of alteration (e.g., acid-sulfate alteration, hydrolytic alteration, hydrothermal alteration at high pH, and high-temperature dry oxidation), and found that acid-sulfate alteration was the only process among those studied that could mobilize the large quantities of Fe from basalts to produce the hematite spherules. By further investigating Fe isotopes of various samples associated with acid-sulfate alteration, and combining the elemental profiles of spherules obtained with NanoSIMS, we reconstructed the history of Fe leaching, precipitation, and mixing processes in relation to spherule formation. The work provides a useful roadmap for analyzing future returned Martian samples.